1. Field of the Invention
This is a metal purification process for zirconium.
This invention relates to high purity zirconium and in particular to electron-beam melting followed by vacuum arc melting.
2. Description of the Related Art
The conventional process for making zirconium metal utilizes a fluidized bed process in which the ore is subjected to a chlorination step which produces a relatively impure, hafnium-containing zirconium tetrachloride and by-product silicon tetrachloride (which by-product is relatively easily separated). The hafnium and zirconium containing material is then subjected to a number of purifying operations and also a complex hafnium separation operation. These operations result in purified oxides of zirconium and hafnium, which, of course, are maintained separate. The purified oxides are separately chlorinated. Zirconium and hafnium are commonly reduced from the chloride by means of a reducing metal, typically magnesium. At the present time, the commercial processes are batch-type processes. U.S. Pat. No. 3,966,460, for example, describes a process of introducing zirconium tetrachloride vapor onto molten magnesium, with the zirconium being reduced and traveling down through the magnesium layer to the bottom of the reactor and forming a metallic sponge. The metallic sponge (containing remaining chloride and some remaining excess reducing metal) is then placed in a distillation vessel for removal of the remaining salt and reducing metal by high temperature vacuum distillation. The sponge material is generally crushed, screened and pressed into electrodes for vacuum arc melting. Particularly, the material is multiple (typical double or triple) vacuum arc melted to provide ingots which are then further fabricated into various shapes. Most of the zirconium currently is used to produce Zircaloy.
Commercial nuclear reactors generally have used Zircaloy tubes as cladding material to contain the uranium dioxide fuel. Generally a Zircaloy ingot is processed into a so-called "trex" and pilgering operations are used to reduce the trex inside diameter and wall thickness to size. Ultrapure zirconium has been proposed for a liner for the inside surface of Zircaloy tubing which is used as a cladding for nuclear fuel and is described in, for example, U.S. Pat. No. 4,372,817 to Armijo et al. on Feb. 8, 1983. A similar use of moderate purity material is proposed in U.S. Pat. No. 4,200,492 to Armijo et al. on Apr. 29, 1980. The ultrapure zirconium material described has been purified by iodide cells to produce so called "crystal bar" material. This rather expensive crystal bar processing is performed after reduction and is described, for example, in U.S. Pat. No. 4,368,072 issued to Siddal on Jan. 11, 1983.
EB (electron-beam) melting of materials, including zirconium has been discussed in a number of patents. EB melting has been used to consolidate crushed particles or chips in so called hearth furnaces and to separate impurities by either overflowing floating inclusions (4,190,404 to Drs et al. on Feb. 26, 1980) or to produce an electrode for arc melting (4,108,644 to Walberg et al. on Aug. 22, 1978). A number of U.S. Patents have used EB melting of powders or granules, often producing an ingot in a chilled mold. These powder melting EB patents include 2,942,098 to Smith on June 21, 1960; 2,960,331 to Hanks on Nov. 15, 1960; 2,963,530 to Hanks et al. on Dec. 6, 1960; 2,997,760 to Hanks et al. on Aug. 29, 1961; 2,935,395 to Smith on May 3, 1960; and 4,482,376 to Tarasescu et al. on Nov. 13, 1984. Electron-beam zone refining using multiple passes is described in U.S. Pat. No. 3,615,345 to King on Oct. 26, 1971.
EB melting using a consumable feed "electrode" to produce an ingot collected in a chilled mold has also been discussed in a number of patents, including 3,087,211 to Howe on Apr. 30, 1963; 3,226,223, to Bussard et al. on Dec. 28, 1965; 2,880,483 to Hanks et al. on Apr. 7, 1959; and 4,130,416 to Zaboronok et al. on Dec. 19, 1978. U.S. Pat. No. 3,219,435 to Gruber et al. on Nov. 23, 1965 shows a commercial type EB furnace utilizing multiple beams. Typically the beams are directed to the surface of the molten pool and are continually swept across the pool surface to avoid overheating of any single portion of the pool surface. U.S. Pat. No. 3,091,525 to D'A. Hunt on May 28, 1963 describes adding a small amount of zirconium, for example, to hafnium, for example and melting in an EB furnace to deoxidize the hafnium. Japanese application No. 1979-144789, published as patent report 1981-67788 by Kawakita describes the use of a very small ingot with a high power density and ultra slow melting to produce a deep molten pool to produce a high purity ingot directly usable for lining of Zircaloy tubing for nuclear reactor applications. Such laboratory sized apparatus with its high power consumption and very low throughput is, of course, not practical for commercial production.